Chalcogenide glasses, those containing one or more of the chalcogenide elements sulfur, selenium, or tellurium, are used in a variety of optical applications. Because of their excellent infrared transmittal properties and relatively low production costs, these glasses, especially selenium-based glasses, are commonly used in infrared optical systems, such as thermal imaging and night vision systems.
Currently, most chalcogenide glasses are produced as small boules in sealed quartz reaction containers. This helps minimize the loss of selenium, which is prone to evaporate out of the reaction melt during the glass formation reaction due to its relatively high vapor pressure. However, production processes such as these suffer from high production costs due to the consumption of the quartz reaction containers, long production times, and environmental risks due to possible explosions.
Once the glass is formed, any practical optical application requires that the glass be formed into an optical component, such as a lens. Current production processes for such optics rely mainly on grinding or molding the chalcogenide glass into lenses. These, however, are rather lengthy processes. After the glass is formed from its constituent elements in a quartz reaction container, the glass is cast into a plate and annealed, so as to avoid breakage. The annealed plate is then cut into blanks, which are ground to thickness, edged, and turned or ground into lenses or formed into lenses in a vacuum press. This process can take several days to complete. Furthermore, production volumes are constrained by the size of the mold ovens used to produce the lenses.
The production of chalcogenide lenses is also hampered by the molds used to form the lenses. Molds typically used for such molding are relatively complex, having a large number of parts and requiring measuring and shimming each time the molds are disassembled for cleaning. Because of their high part count, the tolerance stack-up for each mold prevents molds from being built with acceptable tolerances for high yield processes. Furthermore, reassembly of the molds adds much variability to the part tolerance stack-up, as well.
The molds also suffer from a variety of mechanical problems, as various mold components fail due to the high temperatures they are exposed to during casting and molding. Often times threaded fasteners employed in the molds fail or gall at high temperatures. Furthermore, galling and friction with the mold guide pins frequently lead to mold closing failures.